1. Field of the Invention
The invention relates to a heterojunction bipolar transistor. In particular, it relates to a heterojunction bipolar transistor in which the material composition of the emitter-base heterojunction region is graded, thereby resulting in a high reliability.
2. Description of the Related Art
Conventionally, an AlGaAs/GaAs heterojunction bipolar transistor is formed using a semi-insulating GaAs substrate, doped GaAs base and collector layers and a doped AlGaAs emitter layer. In such a conventional heterojunction bipolar transistor, the material composition of the emitter-base junction region is varied to achieve the desired electric potential barrier. For example, in a device having an emitter-base junction region of which the composition is graded linearly or parabolically, the aluminum content is varied in the emitter-base junction region, namely a transition layer is provided between the emitter layer and base layer as shown by a broken line A in FIG. 4. In the case of a device having an emitter-base junction region of which the composition is graded abruptly, the aluminum content is varied as shown by a solid line B.
Hereinafter, the conventional NPN-type device having an emitter-base junction region of which the composition is graded linearly or abruptly is described in detail with reference to figures. Herein, the term "a grading or abrupt emitter-base junction region" refers to the emitter-base junction region which includes the emitter, transition and base layers and of which the composition is graded linearly, graded parabolically or changes abruptly.
FIG. 5 shows a bipolar transistor having an emitter-base junction region in which the aluminum composition is graded linearly. The bipolar transistor of FIG. 5 has the following layers sequentially formed on a semi-insulating GaAs substrate 31: n.sup.+ -GaAs layer 32, n-GaAs layer 33, p.sup.+ -GaAs layer 34, n-Al.sub.X Ga.sub.l-X As layer 35 (0.ltoreq.X.ltoreq.0.3), n Al.sub.0.3 Ga.sub.0.7 As layer 36 and n.sup.+ -GaAs layer 37. A collector electrode 38 is formed on n.sup.+ -GaAs layer 32. A base electrode 39 is formed on p.sup.+ -GaAs layer 34. An emitter electrode 40 is formed on n.sup.+ -GaAs layer 37.
FIG. 6 shows an energy band of the above described bipolar transistor of FIG. 5 in which the emitter-base junction region has a linearly graded aluminum composition. In the bipolar transistor with the graded emitter-base junction region, a barrier between the emitter and base is released owing to the emitter-base junction region of which the composition is graded linearly or parabolically. In that case, the injected electron density is varied in accordance with the change of applied emitter-base voltage.
FIG. 7 shows a bipolar transistor having an abrupt emitter-base junction region. The bipolar transistor of FIG. 7 has the following layers sequentially formed on a semi-insulating GaAs substrate 51: n.sup.+ -GaAs layer 52, n-GaAs layer 53, p.sup.+ -GaAs layer 54, n-Al.sub.0.3 Ga.sub.0.7 As layer 55 and n.sup.+ -GaAs layer 56. A collector electrode 58 is formed on n.sup.+ -GaAs layer 52. A base electrode 59 is formed on p.sup.+ -GaAs layer 54. An emitter electrode 60 is formed on n.sup.+ -GaAs layer 56.
FIG. 8 shows an energy band of the above described bipolar transistor of FIG. 7 which has the abrupt emitter-base junction region. The bipolar transistor shown in this figure has an emitter-base junction region in which the aluminum composition is changed abruptly, so that electrons come into the base over a barrier from the emitter. In that case, the injected electron density is varied in accordance with the change of applied emitter-base voltage.
The above described bipolar transistors with conventional grading or abrupt emitter-base junction region have carriers (i.e., electrons and holes) at a high density in the emitter-base junction region compared with the collector current density. Accordingly, the possibility of recombination of carriers (SHR recombination) which pass through the energy level of mid gap in this region is quite high. In particular, the possibility is very high at the semiconductor surface where a lot of energy levels exist.
The function of the bipolar transistors of these types are described in detail as follows.
In the case of the NPN-type bipolar transistor with a linear graded emitter-base junction region, a potential well is formed in a conduction band on the emitter side of the emitter-base junction region, so that, there is a reduction in the injection of electrons from the emitter layer into the base layer. Therefore, the electron concentration is increased in the emitter-base junction region.
For a smoothly graded heterojunction at a given collector current density, the electron concentration near the emitter-base junction region is generally quite high because the velocity at which the electrons are injected into the collector is low.
In case of a NPN-type bipolar transistor with an abrupt compositional change at the emitter-base junction region, the concentration of holes in the junction region vicinity becomes very large at a given collector current density owing to the high turn-on voltage of the junction.